use append([]byte, string) special case

[alicebob]

From the specs:

As a special case, copy also accepts a destination argument assignable to type []byte with a source argument of a string type. This form copies the bytes from the string into the byte slice.
https://golang.org/ref/spec#Appending_and_copying_slices

Doesn't seem faster or slower, but is nice to use, and makes it easier for the compiler to be smart.

[valyala]

Thanks for the suggestion. Already done in the commit 4aac41a .

[valyala]

Btw, append(byte, string..) is 2x faster than the previous version according to BenchmarkAppendBytesStr.